The invention relates generally to shutoff valves for wells. In particular, the invention relates to a backup shutoff valve system for a concentric drill string and a method for installing and using the backup shutoff valves in a concentric drill string.
Most oil and gas wells are drilled with a rotary drilling rig. Typically, the drilling rig uses a string of drill pipe with a drill bit on the end. The drill string is rotated by the rotary drilling rig to rotate the drill bit into the ground. A drilling fluid is used to maintain control of the wellbore fluids in the well, as well as to remove the cutting from the wellbore. The drilling fluid may be pumped down the interior passage of the drill pipe, so that it may exit the drill bit through nozzles in the drill bit. The drilling fluid and cuttings are returned to the surface in the annulus space surrounding the drill pipe in the wellbore.
In many cases, the drilling fluid is a liquid known as drilling mud. The density, or weight, of the drilling mud is selected to provide a hydrostatic pressure that is greater than the expected pressure of the fluid in the formation surrounding the wellbore. Consequently, the drilling mud will provide sufficient pressure to prevent a blowout from the well. However, drilling mud can damage the formation around the wellbore, thereby reducing the ability to retrieve fluids later when the well is put into production. For example, methane typically is located in fairly deep coal beds. If liquid drilling mud is used, the coal beds may be damaged by the encroaching drilling mud from the wellbore.
In such circumstances, a gas, such as air, may be used as the drilling fluid. In one such technique, a drill sting with concentric strings of drill pipe: an inner drill string and an outer drill string; is used. The air is pumped down an annulus passage between the two strings of drill pipe. The air and cuttings are returned to the surface within the inner drill string. However, the opposite arrangement may also be used.
Because a column of air does not have the hydrostatic pressure of heavy drilling mud, valve assemblies have been developed to control the flow of fluid within the inner drill string and in the annulus between the inner and outer drill strings in the event that formation fluid begins to flow into the drill string, such as during a blowout or “kick.” However, problems have been experienced with these valve assemblies. For example, these drill string valve assemblies have been known to leak.
Therefore, a more effective technique is desired for preventing an uncontrolled flow of formation fluid upward from the formation to the surface through a concentric drill string. The techniques describe below attempt to solve this problem.